The present invention relates to a telecommunication network for establishing radiofrequency links between gateways and ground terminals via a multispot telecommunication satellite. This type of satellite enables the use of several spot beams from antennas on board the satellite to cover contiguous geographic areas or cells, instead of a single large spot beam.
Such multispot satellites enable several radiofrequency links occupying the same frequency band on different spot beams to be established.
In the case of a high bandwidth broadband satellite telecommunication system, the satellite is used bidirectionally, that is to both:                relay data sent by a gateway (connected to the ground network) to a plurality of ground terminals: this first point to multipoint type link constitutes the forward link;        relay to the gateway data sent by the ground terminals: this second multipoint to point type link constitutes the return link.        
It will be noted that a satellite broadcasting service may be considered to be equivalent to the forward link of a bidirectional system as described above.
An example of a forward link in a multispot telecommunication network is illustrated in FIG. 1.
Signals are sent to a multispot satellite 3 over an uplink LM by a gateway 2 (also called a central station) such as a ground communication gateway connected to an Internet backbone 5. The gateway controls the network through a network management system that allows the operator to monitor and control all the components in the network. The signals sent by the gateway are then processed at the level of satellite 3 that amplifies the signals, transposes the signals at a generally lower frequency and then retransmits the signals from the satellite antenna or antennas on a downlink LD in the form of a plurality of spot beams or spots forming basic coverage areas or cells C1 to C8 in which ground terminals 6 are situated. Each cell C1 to C8 is associated with a spot beam SP1 to SP8. It will be noted that, in the case of configuration 1, the eight cells C1 to C8 associated respectively with eight spot beams SP1 to SP8 form a group of cells served by the same gateway 2. In practice, network 1 is formed by a plurality of gateways that are interconnected via a ground network (an Internet network, for example). The return link of ground terminals 6 to gateway 2 operates identically with a reverse direction of communication.
Coordination of frequencies between operators is done in the context of regulation issued by the International Telecommunication Union (ITU): thus, by way of example, the band Ka for region 1 (Europe, Africa, Middle East) is defined in table 1 below:
TABLE 1Forward linkUplink (from the gateway)27.5 GHz to 29.5 GHzDownlink (to the ground19.7 GHz to 20.2 GHzterminals)Return linkUplink (from the ground29.5 GHz to 30.0 GHzterminals)Downlink (to the gateway)17.7 GHz to 19.7 GHz
It is observed that the spectrums from band Ka in uplink are adjacent (i.e., the intervals [27.5; 29.5] and [29.5; 30.0] do not present any discontinuity). The same is true for spectrums from band Ka in downlink (i.e., the intervals [17.7; 19.7] and [19.7; 20.2] do not present any discontinuity).
Given that the gain from an antenna is inversely proportional to the opening of the spot beam, using multispot antennas to cover an extended area with a homogeneous and elevated gain is necessary. The larger the number of spot beams, the smaller the opening of each spot beam will be. Thus, the gain on each spot beam and so the gain on the service area to cover will be increased. As we mentioned above, a service area to cover is formed by a plurality of contiguous cells (basic coverage areas), one spot beam being associated with each cell. A homogeneous multispot coverage area SA is represented in FIG. 2a), each cell being represented by a hexagon FH such that the coverage area is comprised of a plurality of hexagons FH in which θcell is the outer size of the cell expressed by the angle of the satellite associated with the coverage. However, as the antenna spot beam associated with each cell is not capable of producing a hexagonal form, a good approximation consists of considering a plurality of circular spot beams FC such as represented in FIG. 2b). The association of a spot beam with a cell is done by considering the best performance of the satellite for said spot beam, particularly in terms of EIRP (Equivalent Isotropically Radiated Power) and G/T figure of merit (gain to noise temperature ratio): a cell is determined to be the part of the service area associated with the spot beam that offers the highest gain on this area from among all the satellite spot beams.
Configuration 1 such as represented in FIG. 1 uses a technique known as the frequency reuse technique: this technique enables the use of the same frequency range several times in the same satellite system in order to increase the total capacity of the system without increasing the allocated bandwidth.
Frequency reuse schemes, known as color schemes, making one color correspond to each of the satellite spot beams, are known. These color schemes are used to describe the allocation of a plurality of frequency bands to the satellite spot beams in view of radiofrequency transmissions to carry out in each of these spot beams. In these schemes, each color corresponds to one of these frequency bands.
In addition, these multispot satellites enable the sending (and receiving) of polarized transmissions: the polarization may be linear (in this case the two polarization directions are horizontal and vertical, respectively) or circular (in this case the two polarization directions are left circular or right circular, respectively). It will be noted that in the example from FIG. 1, the uplink leaving the gateway 2 uses two polarizations with four channels for each polarization, respectively Ch1 to Ch4 for the first polarization and Ch5 to Ch8 for the second polarization: the use of two polarizations allows the total number of gateways to be reduced. The eight channels Ch1 to Ch8, after processing by the payload of the satellite 3, will form the eight spot beams SP1 to SP8 (one channel being associated with one spot beam in this example).
According to a four-color scheme (red, yellow, blue, green) with a frequency spectrum of 500 MHz for each polarization, the transmissions being polarized in one of two right circular or left circular polarization directions, each color is associated with a 250 MHz band and a polarization direction.
In the rest of the description, we will take the following convention:
the color red is represented by lines hatched to the right;
the color yellow is represented by dense dots;
the color blue is represented by lines hatched to the left;
the color green is represented by dispersed dots.
A color is thus associated with each satellite spot beam (and thus a cell) such that the spot beams with the same “color” are non-adjacent: contiguous cells thus correspond to different colors.
An example of a four-color scheme for covering Europe is represented in FIG. 3. In this case, 80 cells are necessary to cover Europe.
This type of scheme is applicable equally well in uplink and in downlink. At the satellite level, a spot beam is created from a feedhorn radiating towards a reflector. A reflector may be associated with a color such that four-color coverage is ensured by four reflectors. In other words, the generation of 16 spot beams from each gateway may be done by using four antennas (one per color) each having a reflector, four feedhorns being associated with each reflector.
FIG. 4 illustrates a frequency plan broken down into an uplink frequency plan PMVA on the forward link, a downlink frequency plan PDVA on the forward link, an uplink frequency plan PMVR on the return link and a downlink frequency plan PDVR on the return link. The notations RHC and LHC respectively designate the right and left circular directions of polarization.
The PMVA plan corresponding to the forward uplink (from the gateway to the satellite) disposes 2 GHz (from 27.5 to 29.5 GHz) of available frequency spectrum such that 16 channels of 250 MHz of bandwidth are generated by a gateway (8 channels for each polarization). These 16 channels, after processing by the satellite payload, will form 16 spot beams. The assumption made here consists of considering that the entire 2 GHz spectrum is used: however, it will be noted that it is also possible, particularly for operational reasons, to use only one part of the spectrum and to generate fewer channels. In the example above, 16 spot beams (and thus 16 cells) are generated from two signals multiplexing the 8 channels (a signal multiplexed by polarization) generated by a gateway. Each multiplexed signal corresponding to a polarization is then processed at the satellite transponder level so as to provide 8 spot beams; each of these eight spot beams is associated with a frequency interval among the two frequency intervals [19.7; 19.95] and [19.95; 20.2] and with an RHC or LHC polarization as represented on the downlink frequency plan PDVA.
The PDVR plan corresponding to the return downlink (from the satellite to the gateway) disposes 2 GHz (from 17.7 to 19.7 GHz) of available frequency spectrum such that 16 spot beams of 250 MHz of bandwidth (associated with a frequency interval from among the two frequency intervals [29.5; 29.75] and [29.75; 30.0] and with an RHC or LHC polarization such as represented on the downlink frequency plan PMVR) issued from cells are multiplexed at the satellite level into two signals (corresponding to each polarization) to be returned to the gateway (8 channels for each polarization). We are still assuming that the entire 2 GHz spectrum is used. Thus, in the case of Europe with a spectrum of 2 GHz used, there is a number Nc of cells equal to 80 and a number of active gateways NGWactive equal to 5 (or the number 80 of cells divided by the number 16 of spot beams). It will be noted that It may be that part of the band is not usable, for example the part going from 17.7 to 18.45 GHz in return link and the part going from 27.5 to 28.25 GHz in forward link: in this case, the number of channels Ns per polarization is equal to 5: therefore, the number of cells always being equal to 80 for Europe, the number of active gateways NGWactive becomes equal to 5. In any case, the number of gateways NGWactive is always less than the number Nc of coverage area cells.
For the forward link, each spot beam is associated with one of the four following colors:
a red color corresponding to a first band of 250 MHz (lower part of the available spectrum of 500 MHz) and to the right circular polarization direction;
a yellow color corresponding to the same first band of 250 MHz and to the left circular polarization direction;
a blue color corresponding to a second band of 250 MHz (upper part of the available spectrum of 500 MHz) and to the right circular polarization direction;
a green color corresponding to the same second band of 250 MHz and to the left circular polarization direction; The four adjacent spot beams with the same pattern are each associated with a different color.
On the return link, the polarizations are reversed so that the red and yellow colors have a left circular polarization and the blue and green colors have a right circular polarization. The ground terminals send and receive according to an opposite polarization such that one may easily separate the uplink signals from the downlink signals: such a configuration enables less costly terminals to be used.
The satellite payload designates the part that allows it to fulfill the mission for which it was designed, that is for a telecommunication satellite 3 such as that represented in FIG. 1, to ensure the reception, processing (frequency conversion, filtering, amplification) and resending of telecommunication signals from gateway 2. The payload essentially includes satellite antennas and transponders (and not the equipment for control, propulsion or electrical power equipment which belong to the platform of the satellite).
FIG. 5 represents in a known manner a function block diagram of a payload 10 architecture in forward link (from gateways to cells including ground terminals) with multispot sending over the downlink.
After reception and selection of polarization, 2NGWactive multiplexed signals (in the example cited above, NGWactive signals from 8 channels for each of two polarizations) received from NGWactive gateways (or communication gateway) are each amplified by a 12 LNA low noise amplifier. Each signal is then separated into Nc uplink channels by a signal dividing device (demultiplexer) 13. The Nc uplink channels are then translated in frequency by a frequency converter circuit 14 generally formed by a local oscillator and filtered by a receiver filter 15 (of the band-pass filter type) so as to form Nc channels in agreement with the downlink frequency plan on the forward link (PDVA). The Nc translated frequency channels are amplified through a HPA (High Power Amplifier) high power amplifier 16 generally formed by a CAMP (Channel AMPlifier) channel amplifier 17 and a TWTA (Traveling Wave Tube Amplifier) traveling wave tube amplifier 18 forming Nc downlink spot beam signals. Each of the Nc signals is then filtered through a transmit band-pass filter 19 then sent over a feed 20 such as a feedhorn radiating to a reflector for forming a spot beam. According to this functional configuration, the payload 10 comprises:
2NGWactive LNA low noise amplifiers 12;
2NGWactive signal dividing devices 13;
Nc frequency converter circuits 14;
Nc receiver filters 15;
Nc HPA power amplifiers 16;
Nc transmit band-pass filters 19.
FIG. 6 represents in a known manner a function block diagram of a payload 100 architecture in return link (from cells including ground terminals to gateways) with multispot sending over the uplink.
Nc signals received from Nc cells comprising user terminals are each amplified by an LNA (Low Noise Amplifier) low noise amplifier 112. Each signal is then transposed in frequency by a frequency converter circuit 114 generally formed by a local oscillator and filtered by a receiver filter 115 (of the band-pass filter type) so as to form Nc channels in agreement with the downlink frequency plan on the return link (PDVR). The channels intended for the same gateway (for the same polarization) are then regrouped to form a signal multiplexed by a multiplexer 113 (at Nc inputs and 2NGWactive outputs): the structure of this multiplexed signal is identical to that of a signal sent by a gateway to the satellite on the forward uplink. Thus there are 2NGWactive signals in output from the multiplexer 113. Each of the 2NGWactive signals is amplified through a HPA power amplifier 116 generally formed by a CAMP channel amplifier 117 and a TWTA traveling wave tube amplifier 118 forming 2NGWactive downlink signals in return link. Each of the 2NGWactive return downlink signals is then filtered through a transmit band-pass filter 119 then sent over a radiating device 120 such as a feedhorn radiating to a reflector to form 2NGWactive signals to NGWactive gateways. According to this functional configuration, the payload 100 comprising:
Nc LNA low noise amplifiers 112;
Nc frequency converter circuits 114;
Nc receiver filters 115;
A multiplexer device 113 with Nc inputs and 2NGWactive outputs;
2NGWactive HPA power amplifiers 116;
2NGWactive transmit band-pass filters 119.
It will be noted that channel amplifiers 17 and/or 117 are generally gain control amplifiers that allow the power level of input signals of traveling wave tubes 18 and/or 118 to be adjusted. Tubes 18 and/or 118 may be replaced by SSPA (Solid State Power Amplifier) solid state power amplifiers. It is also possible to use more sophisticated architectures comprising MPA (Multiport Amplifier) type devices offering more flexibility.
However, payloads 10 and 100 such as presented above may pose several difficulties, particularly in the case of TWTA tube breakdowns.
A known solution to this problem consists of using redundant tubes. Such a configuration is illustrated in FIG. 7. FIG. 7 schematically represents the part 200 of a return link payload situated between the multiplexer and the transmit filter and including CAMPs and redundant TWTA tubes.
As explained above, each of the 2NGWactive signals is amplified through an HPA high power amplifier 216 generally formed by a CAMP channel amplifier 217 and a TWTA traveling wave tube amplifier 218 forming 2NGWactive return downlink signals. The difference with FIG. 6 resides in the fact that the payload 200 includes NTWTA HPA power amplifiers 216 formed by NTWTA CAMP channel amplifiers 217 and NTWTA TWTA traveling wave tube amplifiers, NTWTA being strictly greater than 2NGWactive.
The payload 200 comprises first selection means 201 receiving in input the 2NGWactive signals to amplify and selecting 2NGWactive power amplifiers 217 from among the NTWTA present that will perform the amplification. In case of breakdown of a power amplifier (tube failure, for example), it is then possible to use another amplifier by switching the signal on this amplifier. In addition, payload 200 comprises second selection means 202 receiving as inputs the outputs of NTWTA power amplifiers to produce in output 2NGWactive amplified return downlink signals that will then be filtered through an output band-pass filter then sent over a feedhorn radiating towards a reflector to form 2NGWactive signals to NGWactive gateways. Such a configuration allows NTWTA-2NGWactive amplifier failures to be supported. It will be noted that the same type of configuration may be provided in forward link with a redundancy of LNA low noise amplifiers.
However, such a configuration may also present certain difficulties.